1. Field of the Invention
This invention relates to compact zoom lenses of the telephoto type including long focal lengths suited to 35 m/m cameras, video cameras and electronic still cameras and, more particularly, to compact zoom lenses having five lens groups, a plurality of lenses of which are made to move to vary the image magnification, so that the optical performance is kept high over the entire range of magnification of about 2.5.
2. Description of the Related Art
There are many previous proposals for a zoom lens having five or more lens groups, of which at least three lens groups are made to move for zooming purposes in order to achieve a minimization of the entire lens system while securing the predetermined zoom ratio.
Of these, ones which are comprised entirely of five lens groups and have three or more of their lens groups made to move when varying the image magnification are proposed, for example, in Japanese Laid-Open Patent Application No. Sho 60-175020 and Japanese Laid-Open Patent Application No. Sho 63-189819.
The Japanese Laid-Open Patent Application No. Sho 60-1755020 is proposing zoom lenses of the telephoto type including relatively long focal lengths. In this document, either all of the five lens groups having predetermined refractive powers are made to move, or four of them, say, the first, third, fourth and fifth lens groups, are made to move toward, for example, the object side, when the image magnification is varied from the wide-angle end to the telephoto end, thereby providing a range of 3 and a minimum F-number in about 3.6 to 5.6.
Another document or the Japanese Laid-Open Patent Application No. Sho 63-189819 is proposing zoom lenses of the standard form including relatively wide angles of view, when zooming from the wide-angle end to the telephoto end, of the five lens groups having predetermined refractive powers, the first lens group is made to move toward the object side, while the third, fourth and fifth lens groups are made to move in either fixedly or variably spaced relation, as a whole, toward the object side.
In such a way, the zoom lens may entirely be constructed from five lens groups so that a plurality of not less than three of these lens groups can be arranged to move, while maintaining predetermined relations to each other, in order to vary the image magnification. Because this zoom type facilitates a minimization of the size of the entirety of the lens system while securing the zoom ratio at a certain value, it has found its use in the zoom lenses of 35 m/m cameras, video cameras, etc..
With such a zoom type in use, when achieving a further minimization of the bulk and size of the whole lens system by shortening its total length, it is usual to take either of the measures that the number of lens members in each group is reduced, and that the refractive power of each lens group is strengthened.
Relying merely on either or both of the reduction of the number of lens members and the increase of the refractive power of each lens group, however, leads to an insufficiency of aberration correction in each lens group and also an increase of the range of variation of aberrations with variation of the image magnification, thus giving a problem that it becomes very difficult to obtain high grade imagery over the entire range of variation of the image magnification.